Magnesium die casting alloy and process



MAGNESIUM DIE CASTING ALLOY AND PROCESS Filed May 2, 1946 I NVENTOR R/L'HHHU E HflL/SEH ATTORNEY {y for use in die casting.

Patented Mar. 2, 1954 UNITED STATES PATENT- OFFICE MAGNESIUM DIE CASTING'ALLOY AND PROCESS Richard F. Hauser, Bergenfield, N. J assignor to Bendix Aviation Corporation, Teterboro, N. J a corporation of Delaware Application May 2, 1946, Serial No. 666,759

4 Claims.

The present invention relates to a novel magnesium die casting alloy and process for producingsuch an alloy, and is a continuation in part as to all common subject matter of my copending application Serial No. 575,568 filed January 31, 1945, for Magnesium Die Casting Alloy and Process, now abandoned.

An object of the invention is to provide a magnesium alloy having excellent die casting characteristics, whereby both light and heavy sections may be die cast with slight tendency to- .wards hot cracking during the die casting process and only moderate tendency towards internal porosity in the heavy sections.

Another object of the invention is to provide a magnesium alloy having superior corrosion resistance due in part to its manganese content and also to its low impurity content caused by the settling out in large part of such impurities through the nove1 method employed in preparing the alloy.

Another object of the invention is to provide a novel process for preparing a magnesium al- Another object of the invention is'to provide a novel method of preparing a magnesium alloy in which there is added to a magnesium-manganese alloy an amount of aluminum to decrease the solubility of the manganese in the alloy so as to precipitate a portion of the manganese out of solution tending to cleanse the melt of suspended iron particles while the temperature-of the melt is maintained sufiiciently high so as to prevent reduction of the manganese below a predetermined desired value.

Another object of the invention is to provide 7 a process for causing an equi-axed grain structure to occur with resultant improved ductility and stability to the material. By the term equiaxed is meant the approximate equal dimensions of the grain with relation to their own axes and in relation to each other.

Other objects and advantages of this invention 4 are set forth in the following description and the novel features thereof are pointed out in the appended claims. The disclosure, however, is il lustrative only and changes may be made therein within the principle of the invention to the full extent indicated by the broad and general meanings of the terms in which the appended claims are expressed.

In Figure 1, there is illustrated a photo-micrograph of a typical grain structure obtained in this alloy in the as cast condition and before aging. In Figure 2, there is illustrated a photo-micrograph of a typical grain structure obtained in this alloy after the heat treatment or artificial aging process explained hereinafter and forming the subject matter of the present application. 1 I v Specifically, as an example of my invention, the alloy is prepared by melting a binary magnesium-manganese alloy containing approximately 1.5% manganese by weight, and to the molten metal adding'l.00% to 1.5% aluminum byweight at approximately 1250-1350 degrees Fahrenheit.

The addition of this amount of aluminum will decrease the solubility of the manganese in the magnesium to between .5% and 1.0% by Weight. The melt is held at this temperature for an appreciable period depending upon size of melt, for example, a minimum of fifteen minutes for a thousand pound melt; so that the manganese which is precipitated out of solution tends to cleansethe melt of suspended iron and other particles by causing them to sink to the bottom of the melt due to agglomeration of these particles to the precipitated primary manganese. This action is not to be confused with the heretofore known settling process in which the precipitation of manganese depends primarily upon holding the melt for long periods of time at relatively low temperatures. In the instant process the temperature of the alloy should never be allowed to drop below 1250 degrees Fahrenheit before being cast or frozen, as such a decrease in temperature below 1250 degrees Fahrenheit during the settling process might cause an adverse reduction of the manganese below the specified desired minimum of .5%. It is important to retain at least this amount of manganese in order to still further increase the "corrosion resistance of the alloy. v

The magnesium-alloy thus obtained will have an aluminum content by weight of between 1.0% to 1.5 a manganese content by weight of not less than a .5% minimum and impurities or other ingredients by weight not in excess of a .3% maximum. v r Upon completion of the settling process-the thus-cleansed magnesium-manganese-aluminum alloy is removed and may be directly die cast or may be cast into pigs for convenience in handling. In the latter event, the pigs, as desired, are reheated to atemperature of between 1250 degrees and 1350 degrees Fahrenheit for pouring into a die casting mechanism of conventional type.

- Due to the short time for diffusion during solidification, sufficient deviation from equilibrium conditions are attained to permit a small amount .of theharclening materia1-or eutectic to be retained atithe grainiboundarie's ofv the solidified metal. Precipitation of the eutectic, an aluminum magnesium compound, is later ac= centuated 'by a novel artifical aging process explained hereinafter. However, the: amount of this material in the grain boundaries is not suni cient to cause excessive hot shortness or embrittlement of the die castings atielevated terns peratures, since the aluminum content is prefer.- ably maintained at a value not in excess of 1.5% by weight. It has been found.- that' with an aluminum content in excess of 1.5%, hot-shortness or embrittlement at elevated temperatures becomes so pronounced-that it would be very 'dimcult to make" certain die castings without hot-cracking occurring as the casting is ejected from :thedie.

vvItjis important, however, that the" aluminum 'content'of the alloy'b'e maintainedat a mini-- mum'of 1.0% or good mechanical properties will 'notbeobtained' in'the subsequent die castings after the heat treatment or aging processand internal'porosity will be increased to such an extent as'to cause'the castings to be unsound;

This alloy has the following composition:

Aluminum 1.0-1.5% -Manganese-- 0.5-1.0% Nickel 0.005%max. Iron 0.005% max. Copper 0.05%max. Silicon 1 0.03% max. Total elements other. than .Mg, Al,

and Mn 0.3 max. Magnesium Balance I Theab'ove alloy is-then die cast andit has been found' that'by subjecting the die casting to a higher annealing temperature than customarily used'for annealing or aging, and reducing the time at suchtemperature, a maximum amountof precipitation of the intermetallicchemical'compound of aluminum-magnesium could be obtained; This-resulted in increased yield strength and-hardness of castings of this alloy so treated,

equi axedzstructure as?shown for'example. in the drawingof Figure 2 which materially improves the mechanical properties of the. material.

The comparative mechanical properties of the sameiare: as. follows:

Heat AS Cast Treated Condition 4 hrs. at

Tensile Strength, p. s. i 30, 000 32, 000 Yield Strength.-. 8, 000 21,000 Elongation, pcrcen l6 9 Impact, loot lbs l4 l0 Hardness RE; -l 35, 65

Yield strength is defineclasthestressat which thev-stressestrain curve-deviates: 2%? from the modulus line.

From the foregoing it will be readily seen that there has been provided a novel improved magnesiunr. alloy? and novel heat treating or aging process for producing the same.

Whileonly one.form of the invention is illustrated'herein, it isto be clearly understood that the inventionisnot to be limited thereto, but the same is to be considered merely as illustra tive of'theipraetice thereof, since the invention is nottobe limited except by the appended claims.

What is claimed is:

1; A die cast magnesium alloyconsistingof between 113% and 1.5% by weight of aluminum; between 0.5% and'1.0% by weightof manganese; total elements other than magnesium, manganese and aluminum not in excessof .3%-by weight, and the-balance of magnesium, said alloy having a substantially homogeneous equi axed grain structure.

2. A die cast magnesium-alloy consisting of between about 1.0%and about 1.5% by weight of aluminum; total elements other than-magnesiumand aluminum not in excess of about 1.3% by weight, and the balance of magnesium, said alloy having a substantially homogeneous equi-axed grain stucture.

3. A process for heat treating a die casting of a magnesium base alloyto improve the grain structurethereoi, in which said magnesium alloy contains between 1.0% to 15% by weight' of aluminum and the balance substantially of magnesium; comprising subjecting thescasting' to-a temperature of approximately 475-' F. for approximately four hours.

4. A process for heat treating a die casting-of a magnesium base alloy' to improve the grain structure thereof, inwhich said'magnesium alloy contains between 1.0% to 1.5% by weight of aluminum and the balance substantially of magnesium; comprising'subiecting the casting to heat treatment at'a temperature'of from about 450'to about 500 F. for approximately three tofi-'ve hours.

RICHARD F. HAUSERi References Cited in .thesfil'efof this: patent."

, UNITED STATES PATENTS Number.

(Provisional and amended) OTHER REFERENCES Die'Casting, by-Herb, page 51, 1936.

Metals Handbook, page 1583, Am. Soc. for Metals, 1939.

Die Casting; for Engineers, pub. by New Jersey Zinc, pages 28-31, 1942.

Magnesium, pages 67 and 68. Am. Soc. for Metals; 1946. 

3. A PROCESS FOR HEAT TREATING A DIE CASTING OF A MAGNESIUM BASE ALLOY TO IMPROVE THE GRAIN STRUCTURE THEREOF, IN WHICH SAID MAGNESIUM ALLOY CONTAINS BETWEEN 1.0% TO 1.5% BY WEIGHT OF ALUMINUM AND THE BALANCE SUBSTANTIALLY OF MAGNESIUM; COMPRISING SUBJECTING THE CASTING TO A TEMPERATURE OF APPROXIMATELY 475* F. FOR APPROXIMATELY FOUR HOURS. 